In solar cells, liquid crystal display (LCD) units, plasma display (PDP) units and the like of the related art, a base material with a transparent conductive film (TCF) that is obtained by forming the transparent conductive film on the transparent base material that is formed, for example, from glass or the like that is a non-conductive body is widely used.
These transparent conductive films are films whose main constituent is a conductive metallic oxide such as indium tin oxide (ITO), tin oxide (TO), and fluorine-doped tin oxide (FTO), and have a combination of excellent transparency to visible light and excellent electrical conductivity. Among these transparent conductive films, transparent conductive films having indium tin oxide (ITO), in particular, as their main constituent are widely known, and these are used in the liquid crystal display (LCD) units for personal computers (PC), televisions, and mobile telephones and the like.
One method of forming a transparent conductive film such as indium tin oxide (ITO) on a transparent base material is spray pyrolysis deposition (SPD).
This spray pyrolysis deposition is a technology involving a series of reactions. In this technology, a solution constituting a raw material is sprayed using a spraying device such as an atomizer onto a base material that has been preheated to a film formation temperature. In the initial stages of the resulting reaction, crystals are formed as a result of a vaporization of the solvent contained in the droplets that have been deposited on the surface of the base material and a reaction of solutes in the droplets. As the reaction progresses, droplets adhere onto the crystals (i.e., a polycrystalline substance) that have formed on the base material, and, as a result of a vaporization of the solvent in the droplets and a progress of the reaction between the solutes and the crystals underneath, crystalline (i.e., a polycrystalline substance) growth progresses.
In this spray pyrolysis deposition, an aqueous solution or alcohol solution of a metal inorganic salt, or an organic solution obtained by dissolving an organic metal compound or organic acid-base in an organic solvent, or a mixed solution obtained by mixing these solutions, or the like is used as the favorable raw material solution to be sprayed. The temperature of the base material differs depending on the type of starting material or raw material solution, however, the temperature range is set to 250 to 700° C. Because the film forming apparatus used in this type of spray pyrolysis deposition is simple and low in cost, it is effective when forming transparent conductive films at low cost.
A transparent conductive film (TCO: transparent conductive oxide) is glass that has been provided with conductivity by forming a thin film of a semiconductor ceramic such as tin-doped indium oxide (ITO), tin oxide (TO), or fluorine-doped tin oxide (FTO) on the surface of non-conductive glass, and has the property of conducting electricity in spite of being transparent. Among these, ITO, in particular, is widely known as a transparent conductive film and is used in the liquid crystal display units of personal computers, televisions, and mobile telephones and the like.
Using spray pyrolysis deposition, it is possible to form a transparent conductive film or the like at low cost because the film forming apparatus is simple and the raw material is also comparatively low in cost. An aqueous solution or alcohol solution of a metal inorganic salt, or an organic metal compound or organic solvent based solution of an organic acid-base is used for the starting material of the transparent conductive film. The temperature of the substrate differs depending on the starting material or raw material solution, however, the temperature range is set to 250 to 700° C.
However, in a related art film forming apparatus 1100 that includes a liquid supply component 1120 and a vapor supply component 1121 such as is shown in FIG. 1, in a fine particle formation device a, liquid that is supplied from the liquid supply component 1120 and vapor that is supplied from the vapor supply component 1121 are made to collide with each other so that the raw material solution formed by the two is changed into fine particles. When the raw material in fine particle form is sprayed onto a base material 1110 by a spray device c, the size of droplets 1122 that are sprayed from the spray device c is dependent on the spray nozzles (may also be hereinafter referred to as two-fluid spray nozzles) in the spray device c, and it is difficult to obtain a uniform size in the droplets 1122 which causes the film thickness to be uneven. Namely, in the preparation of a transparent conductive film using spray pyrolysis deposition inside a transparent conductive film forming device, when spray nozzles are used to spray a raw material solution onto a base material that has been heated to a temperature range of 250 to 700° C., the droplets 1122 that are sprayed from the spray nozzles have a size distribution of between 10 μm and 120 μm, as is shown by the related art apparatus in FIG. 6, even when two-fluid spray nozzles that allow fine particles to be formed are used. As a result, when forming a film over a large surface area, in-plane distribution ends up being generated in the sprayed droplets (i.e., mist) so that film thickness distribution is increased and a considerably high distribution is created in some film characteristics, such as sheet resistance and transmissivity.
Therefore, several devices have been proposed as devices to make the size of the sprayed droplets uniform. For example, it has been observed that, among droplets sprayed from spray nozzles, droplets having a large particle diameter are present in greater numbers at positions away from the center of the spray path. It has also been observed that coarse droplets contained in the vicinity of the center have a fast spray speed and fly further than fine droplets. Accordingly, technology has been proposed in which wall surfaces are provided at a front surface and surrounding the spray path so that droplets having a large particle diameter that fly to positions away from the center of the spray path and coarse droplets that fly far from the vicinity of the center collide with these wall surfaces and are removed (see Japanese Unexamined Patent Application, First Publication No. H05-320919 and Japanese Unexamined Patent Application, First Publication No. 2001-205151).
However, the above described devices attempt to make the size of the droplets uniform by efficiently selecting sprayed droplets, and do not spray droplets whose particle sizes have already been made uniform in advance. Accordingly, there is a limit as to how uniform the size of the droplets can be made and it is difficult to use only fine droplets to form a film.
Moreover, it is necessary for a sufficient distance, for example, approximately 500 mm to be provided between the discharged spray and the base material. This makes positive temperature control as well as control of the droplet spray speed and of the force of their collision with the base material impossible. As a result, film formation in which the film characteristics are precisely controlled has not been possible.
Another example of a related film forming apparatus that uses spray pyrolysis deposition is shown in FIG. 2. This film forming apparatus 2100 comprises a supporting device 2120 on which a substrate 2110 is mounted and with a discharge device 2130 that sprays a raw material solution in spray form. The supporting device 2120 has a heating device embedded therein that heats a mounted substrate to a predetermined temperature (see, for example, Japanese Utility Model Application No. H06-012446).
In order to spray mist uniformly onto a substrate having a large surface area at an angle of, for example, 200 mm or more, it is necessary to arrange and drive a large number of mist spray nozzles.
However, if mist is sprayed using related art circular nozzles (i.e., 60φ mm), then as is shown in FIGS. 3A and 3B, if the nozzles are driven in a circular shape or, as is shown in FIGS. 4A and 4B, if the nozzles are driven in an elliptical shape, the distribution is increased in the sprayed quantity of the mist. In order to reduce the effects of this distribution, it has been necessary to conduct even more complex drive control.